1. Field of the Invention
The present disclosure relates generally to computer networks and more particularly to efficiently decoupling reservation and data forwarding of a data flow in a computer network.
2. Background Information
A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between end nodes, such as personal computers and workstations. Many types of networks are available, with the types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect the nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), or synchronous digital hierarchy (SDH) links. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). In this context, a protocol consists of a set of rules defining how the nodes interact with each other. Computer networks may be further interconnected by an intermediate network node, such as a router, to extend the effective “size” of each network.
Since management of interconnected computer networks can prove burdensome, smaller groups of computer networks may be maintained as routing domains or autonomous systems. The networks within an autonomous system (AS) are typically coupled together by conventional “intradomain” routers configured to execute intradomain routing protocols, and are generally subject to a common authority. To improve routing scalability, a service provider (e.g., an ISP) may divide an AS into multiple “areas” or “levels.” It may be desirable, however, to increase the number of nodes capable of exchanging data; in this case, interdomain routers executing interdomain routing protocols are used to interconnect nodes of the various ASes. Moreover, it may be desirable to interconnect various ASes that are operated under different administrative domains. As used herein, an area or level, or more particularly, an AS, is generally referred to as a “domain,” and a router that interconnects different domains together is generally referred to as a “border router.”
Examples of an intradomain routing protocol, or an interior gateway protocol (IGP), are the Open Shortest Path First (OSPF) routing protocol and the Intermediate-System-to-Intermediate-System (IS-IS) routing protocol. The OSPF and IS-IS protocols are based on link-state technology and, therefore, are commonly referred to as link-state routing protocols. Link-state protocols define the manner with which routing information and network-topology information are exchanged and processed in a domain. This information is generally directed to an intradomain router's local state (e.g., the router's usable interfaces and reachable neighbors or adjacencies). The OSPF protocol is described in RFC 2328, entitled OSPF Version 2, dated April 1998 and the IS-IS protocol used in the context of IP is described in RFC 1195, entitled Use of OSI IS-IS for routing in TCP/IP and Dual Environments, dated December 1990, both of which are hereby incorporated by reference.
An intermediate network node often stores its routing information in a routing table maintained and managed by a routing information base (RIB). The routing table is a searchable data structure in which network addresses are mapped to their associated routing information. However, those skilled in the art will understand that the routing table need not be organized as a table, and alternatively may be another type of searchable data structure. Although the intermediate network node's routing table may be configured with a predetermined set of routing information, the node also may dynamically acquire (“learn”) network routing information as it sends and receives data packets. When a packet is received at the intermediate network node, the packet's destination address may be used to identify a routing table entry containing routing information associated with the received packet. Among other things, the packet's routing information indicates the packet's next-hop address.
To ensure that its routing table contains up-to-date routing information, the intermediate network node may cooperate with other intermediate nodes to disseminate routing information representative of the current network topology. For example, suppose the intermediate network node detects that one of its neighboring nodes (i.e., adjacent network nodes) becomes unavailable, e.g., due to a link failure or the neighboring node going “off-line,” etc. In this situation, the intermediate network node can update the routing information stored in its routing table to ensure that data packets are not routed to the unavailable network node. Furthermore, the intermediate node also may communicate this change in network topology to the other intermediate network nodes so they, too, can update their local routing tables and bypass the unavailable node. In this manner, each of the intermediate network nodes becomes “aware” of the change in topology.
Typically, routing information is disseminated among the intermediate network nodes in accordance with a predetermined network communication protocol, such as a link-state protocol (e.g., IS-IS, or OSPF). Conventional link-state protocols use link-state advertisements or link-state packets (or “IGP advertisements”) for exchanging routing information between interconnected intermediate network nodes (IGP nodes). As used herein, an IGP advertisement generally describes any message used by an IGP routing protocol for communicating routing information among interconnected IGP nodes, i.e., routers and switches. Operationally, a first IGP node may generate an IGP advertisement and “flood” (i.e., transmit) the packet over each of its network interfaces coupled to other IGP nodes. Thereafter, a second IGP node may receive the flooded IGP advertisement and update its routing table based on routing information contained in the received IGP advertisement. Next, the second IGP node may flood the received IGP advertisement over each of its network interfaces, except for the interface at which the IGP Advertisement was received. This flooding process may be repeated until each interconnected IGP node has received the IGP advertisement and updated its local routing table.
In practice, each IGP node typically generates and disseminates an IGP advertisement whose routing information includes a list of the intermediate node's neighboring network nodes and one or more “cost” values associated with each neighbor. As used herein, a cost value associated with a neighboring node is an arbitrary metric (a “link metric”) used to determine the relative ease/burden of communicating with that node. For instance, the cost value may be measured in terms of the number of hops required to reach the neighboring node, the average time for a packet to reach the neighboring node, the amount of network traffic or available bandwidth over a communication link coupled to the neighboring node, etc. Notably, as those skilled in the art will understand, Traffic Engineering (TE) extensions to IGP advertisements may be used to convey various link metrics, such as, e.g., link utilization, etc. Examples of TE extensions for IGP can be found in RFC 3784 entitled Intermediate-System-to-Intermediate-System (IS-IS) Extensions for Traffic Engineering (TE) dated June 2004, and RFC 3630, entitled Traffic Engineering (TE) Extensions to OSPF Version 2 dated September 2003, the contents of both of which are hereby incorporated by reference in their entirety.
As noted, IGP advertisements are usually flooded until each intermediate network IGP node has received an IGP advertisement from each of the other interconnected intermediate nodes. Then, each of the IGP nodes (e.g., in a link-state protocol) can construct the same “view” of the network topology by aggregating the received lists of neighboring nodes and cost values. To that end, each IGP node may input this received routing information to a “shortest path first” (SPF) calculation that determines the lowest-cost network paths that couple the intermediate node with each of the other network nodes, i.e., thus computing a “shortest path tree” (SPT), as will be understood by those skilled in the art. For example, the Dijkstra algorithm is a conventional technique for performing such an SPF calculation, as described in more detail in Section 12.2.4 of the text book Interconnections Second Edition, by Radia Perlman, published September 1999, which is hereby incorporated by reference as though fully set forth herein. Each IGP node updates the routing information stored in its local routing table based on the results of its SPF calculation. More specifically, the routing table and, in turn, the forwarding table(s) are updated to correlate destination nodes with next-hop interfaces associated with the lowest-cost paths to reach those nodes, as determined by the SPF calculation.
The data packets transferred among the network nodes may include fixed-sized data cells and/or variable-sized data frames. Each data packet typically comprises “pay-load” data prepended (“encapsulated”) by at least one network header formatted in accordance with a network communication protocol. The network headers include information that enables the client nodes and intermediate nodes to route the packet efficiently through the computer network. Often, a packet's network headers include at least a datalink (layer 2) header and an internetwork (layer 3) header, as defined by the Open Systems Interconnection (OSI) Reference Model. The OSI Reference Model is generally described in more detail in Section 1.1 of the reference book entitled Interconnections Second Edition, by Radia Perlman, published September 1999, which is hereby incorporated by reference as though fully set forth herein.
In operation, a client node may send a data packet to a network interface of an intermediate network node. Thereafter, the intermediate network node receives the packet and forwards the packet to its next destination. For example, the intermediate network node may perform a layer-2 switching function that simply re-directs the packet from one network interface to another based on the contents of the packet's data-link header. Alternatively, the intermediate network node may perform a layer-3 routing function, or forwarding decision, that selects the most appropriate network interface to forward the packet based on the contents of the packet's internetwork header.
Data packets are used to transport many forms of information, including voice and video information, over networks and subnetworks. For instance, voice information may be transmitted in accordance with the Voice over Internet Protocol (VoIP). VoIP refers to a group of technologies used to transmit voice information over data networks from a source node to a destination node. The source and destination nodes employ voice agents that convert voice information from its traditional telephony form to a form that is suitable for packet transmission. In other words, the source node's voice agent encodes, compresses, and encapsulates the voice information into a plurality of data packets, and the destination node's voice agent performs complementary functions to de-encapsulate, uncompress, and decode the VoIP packets. Examples of voice agents include IP telephones, VoIP gateways, certain private branch exchanges (PBXs), personal computers (PCs) running communication applications, network devices providing voice gateway services, etc. Also, video information may be transmitted in accordance with Video on Demand (VoD) standards known to those skilled in the art in a similar manner. For instance, a VoD content server may supply video data streams to one or more “set-top-boxes” of users. Notably, the use of VoIP and VoD are examples of applications (e.g., at an application level) that a node within the network may operate. Those skilled in the art will understand that other applications may also be operated at network nodes.
A source node (sender) may be arranged to transfer a unidirectional stream of data packets, or a “data flow,” to a destination node (receiver) in a data network. The data flow may comprise, for example, data or voice/video information. The data flow is unidirectional in that data travels one-way from the sender to the receiver. The logical procession of intermediate network nodes that transmit and receive data packets from the sender to the receiver defines the data flow's data path. A first node that is nearer the receiver in the data flow's data path than a second node in the flow is said to be “down-stream” from the second node. Likewise, a first node that is nearer the sender in the data flow's path than a second node in the flow is said to be “upstream” from the second node.
Some data flows are associated with a certain level of quality of service (QoS). For example, a data flow's QoS may specify minimum end-to-end latency or bandwidth requirements needed to support the flow. The Resource ReSerVation Protocol (RSVP) is a network-control protocol that enables source and destination nodes to “reserve” the necessary resources to establish the data flow in accordance with the flow's required QoS. RSVP works in conjunction with routing protocols to, e.g., reserve resources along a data flow between the source and destination nodes to establish a level of QoS required by the data flow. RSVP is defined in R. Braden, et al., Resource ReSerVation Protocol (RSVP), Request For Comments (RFC) 2205, which is hereby incorporated by reference as though fully set forth herein.
In a typical arrangement, the sender sends an RSVP Path message identifying the flow (e.g., by identifying the sender address, the receiver address, source and destination port numbers, etc.), and indicating the bandwidth needed by the data flow. The Path message proceeds to the receiver following the flow's data path and each intermediate network node may update an optional “Adspec” object of the Path message. The Adspec object contains, inter alia, information about the characteristics of the network on the data flow path, such as available services, QoS, delay, and bandwidth estimates. Adspec objects may be generated by senders or by intermediate nodes, and are modified as they travel from one node to another. An Adspec object advertises the possible service parameters composed of the properties of all previous-hop nodes upstream. Namely, the arriving Adspec object is combined with the node's own parameters and service conditions, and then forwarded to the next node. A receiver can use the Adspec information to predict the end-to-end QoS, to choose the most appropriate service and to scale its QoS request according to the current possibilities of the network.
The receiver receives the Path message and may take into account contents of the optional Adspec object in the Path message to determine the specifics of the reservation requests it will generate for the flow. The receiver generates a “request for resources” in the form of an RSVP reservation request (Resv message) which travels hop-by-hop back to the sender. Within the Resv message is a “FlowSpec” object, which contains, inter alia, an indication of a peak expected traffic (e.g., bandwidth) from the sender (Tspec), and the requested traffic value to be reserved (Rspec). At each hop, the corresponding intermediate network node sets aside (“assigns”) sufficient resources to provide the requested bandwidth for the desired data flow. These assigned resources are consequently made available to the data flow so that the data packets of the flow get appropriate QoS treatment (i.e., the data flow is “admitted”).
If sufficient resources are not available, an intermediate network node may “reject” the Resv message (i.e., does not continue forwarding it), generate a reserve error (ResvErr) message, and forward the ResvErr message downstream towards the receiver. The ResvErr message travels downstream hop-by-hop and the receiver eventually receives the ResvErr message and concludes that the reservation has failed. A receiver whose Resv message has been rejected may attempt to acquire less resources by sending another Resv message requesting less bandwidth, or the receiver may re-attempt later to acquire the resources by re-transmitting another Resv message. Senders are unaffected by this process, and they continue to send Path messages to refresh their state. Notably, PathErr and ResvErr messages are typically sent and processed hop-by-hop by the intermediate nodes along the path between the sender and the receiver, as will be understood by those skilled in the art.
Notably, the reservation and data forwarding of the data flows are “coupled,” in that they conventionally follow the same path. For instance, the path used for the forwarding of data flow data between the sender and the receiver is conventionally the current best path as selected by the routing protocols (e.g., IGP), such as the shortest (best) path of an SPF calculation. In that case, the path used to establish the reservation for the data flow is also the current best path as selected by the routing protocols. In the event the current path does not have sufficient resources for the data flow, the reservation is rejected, even if another (e.g., a non-shortest) path may have sufficient available resources. In essence, this practice is wasteful of available resources on non-best (non-shortest) paths, as available resources in the network remain unutilized. In particular, a data flow reservation may be unnecessarily rejected based on a fully reserved best path, while an acceptable non-best path may have available resources for the data flow. Also, it may often be the case that better (shorter) paths are fully reserved by variable rate data flows (i.e., reserved based on peak expected traffic), yet those variable rate data flows are not fully utilizing their reserved bandwidth. Other data flows may not be able to reserve bandwidth on the better (shorter) paths, even where those paths have sufficient unutilized (even though reserved) bandwidth.
One solution to allow IP routing to use non-shortest paths involves identifying a path with sufficient reservable resources, reserving that path, and programming states in each forwarding node along the path to ensure that data from the data flow will be forwarded along that reserved path. Notably, the path with sufficient reservable resources (e.g., bandwidth) need not be the shortest path, but may be a non-shortest path. Accordingly, per-flow states are necessary in order to forward the data (e.g., IP routed data) along non-shortest paths, where the per-flow states of each node along a path correlate the particular data flow and the appropriate next-hop node (e.g., not necessarily the next-hop node for the best/shortest path as determined and generally utilized by the routing protocols). These per-flow states, however, pose a scalability problem when large numbers of reservations are maintained at a node (e.g., a border router). Also, the reservation and forwarding of the data flows, though on non-shortest paths, remain coupled on the same path.
In addition, various mechanisms have been proposed to allow dynamic redistribution of data flows in a network using shortest and non-shortest paths based on current load, in order to avoid congestion that would otherwise exist if only the shortest paths were used. However, current reservation protocols would not operate properly in conjunction with these mechanisms, since the reservation protocols would always be trying to keep track of current best paths for all data flows based on the changing load, and attempting to reestablish the reservations along the best paths. Moreover, this dynamic redistribution would result in extraneous signaling and processing load.
There remains a need, therefore, for a technique that allows reservations to be placed on paths other than the current forwarding path, and for data associated with reserved data flows to be forwarded on paths other than the reserved path. In addition, there remains a need for this “decoupling” of the reservation and data forwarding of the data flow to be available without requiring per-flow states in the forwarding path, and without excess signaling and/or processing load.